Such a brake power assist unit and its respective application in regulating the brake pressure is known from DE 4,324,205 A1. In this case, the position of a sleeve is controlled by means of a position regulator that receives signals from a displacement transducer and prevents an excessive destabilization of the load characteristic by controlling the current delivered to the electromagnet in such a way that a stable position of the sleeve is attained. In this brake power assist unit, a pressure regulator is connected in series with the position regulator, wherein the output signal of the pressure regulator corresponds to a nominal value position for the ancillary position regulator.
The present invention is based on the objective of proposing measures which allow a stabilization of the load characteristic during the pressure reduction phase due to constructive modifications on the control valve of the brake power assist unit that can be externally controlled. The invention, in particular, aims to eliminate the need for the aforementioned costly position regulator as well as the sensor arrangement.
According to the invention, this objective is attained by providing means that allow a controlled pneumatic pressure compensation between the operating chamber and the vacuum chamber or a reduction of the force to be generated by the electromagnet during the pressure reduction phase, i.e., during the return movement of the third sealing seat from the valve body.
According to one advantageous additional refinement of the invention, a ring that is arranged such that it can be moved relative to the third sealing seat is supported on the sleeve by way of an elastic or compressible element, wherein the edge region of the ring which faces the valve body is provided with openings and arranged axially offset relative to the third sealing seat, namely toward the valve body, in the separated position.
In another advantageous embodiment of the invention, the valve body is provided with an additional annular surface that contains radial openings within the region in which it contacts the third sealing seat.
In another alternative embodiment, the third sealing seat is provided with axial recesses or depressions that, for example, may be realized in the form of a corrugated sealing edge.